Theory of Machine
The static balancing is satisfactory for low speed rotors but with increasing speeds, dynamic balancing becomes necessary. This is because, the

Effects of unbalances are directly proportional to the speed
Unbalanced forces are not dangerous at higher speeds
Unbalanced couples are caused only at higher speeds
Effects of unbalances are proportional to the square of the speed

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Theory of Machine
Two pulleys of radii r₁ and r₂ and at distance x apart are connected by means of an open belt drive. The length of the belt is

π (r₁ + r₂) + (r₁ - r₂)²/x + 2x
π (r₁ - r₂) + (r₁ + r₂)²/x + 2x
π (r₁ - r₂) + (r₁ - r₂)²/x + 2x
π (r₁ + r₂) + (r₁ + r₂)²/x + 2x

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